Magnetic-disk substrate, magnetic disk, and magnetic-disk drive device

ABSTRACT

A magnetic-disk substrate has a pair of main surfaces, and an arithmetic average roughness Ra of each of the main surfaces is 0.11 nm or less. The arithmetic average roughness Ra is a value obtained through measurement using an atomic force microscope provided with a probe having a probe tip provided with a carbon nanofiber rod-shaped member. The magnetic-disk substrate is made of glass or aluminum alloy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.16/395,850, filed on Apr. 26, 2019, which is a continuation applicationof U.S. patent application Ser. No. 15/545,969, filed on Jul. 24, 2017,now U.S. Pat. No. 10,319,403, which is a U.S. National stage applicationof International Patent Application No. PCT/JP2016/060647, filed on Mar.31, 2016, which, in turn, claims priority under 35 U.S.C. § 119(e) toU.S. Provisional Patent Application No. 62/141,121, filed on Mar. 31,2015, and the entire contents of U.S. patent application Ser. Nos.16/395,850 and 15/545,969, International Patent Application No.PCT/JP2016/060647, and U.S. Provisional Patent Application No.62/141,121 are hereby incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to a magnetic-disk substrate having a pairof main surfaces, a magnetic disk, and a method for manufacturing amagnetic-disk substrate.

Background Information

Conventionally, an aluminum alloy substrate or a glass substrate hasbeen preferably used in a magnetic disk used as one type of informationrecording medium. Nowadays, in order to meet the demand for an increasein the storage capacity of hard disk drive apparatuses, attempts havebeen made to increase the magnetic recording density. Accompanying this,the magnetic recording information area has been made smaller by makingthe flying height of the magnetic head from the magnetic recordingsurface extremely low. For example, magnetic recording is performed byforming perpendicular magnetization in a magnetic layer of the magneticdisk. In such a magnetic-disk substrate, in order to achieve a lowerflying height of the magnetic head, which is necessary for ahigh-recording density hard disk drive apparatus, a decrease in surfaceunevenness of the magnetic-disk substrate is in increasing demand.

For example, there is known to be a substrate for a perpendicularmagnetic recording medium, which is a substrate for a magnetic recordingmedium, is made of a nonmagnetic material, and has an inclination anglein a surface profile curve of 2 degrees or less, or has a surface shapein which a surface roughness Ra is 0.15 nm or less in a range from asurface roughness Ra having a cycle of 83 nm or less to a surfaceroughness Ra having a cycle of 30 nm or less (JP 2008-293552A). It isassumed that according to this substrate, it is possible to improve thecrystal orientation of magnetic particles formed on this substrate andachieve a decrease in noise of a recording layer (or a magnetic layer)of the magnetic recording medium.

SUMMARY

The surface roughness Ra of the above-described substrate for aperpendicular magnetic recording medium is obtained based on the resultsof performing measurement with an atomic force microscope. The surfaceroughness is measured with the atomic force microscope, using a membermade of single crystal Si as a probe tip, for example. However, even ifthe surface roughness Ra was 0.15 nm or less, magnetic-disk propertieswere not necessarily improved in some cases. For example, a BER (biterror rate) can be obtained as a magnetic-disk property by reading out asignal obtained after the signal is recorded in a magnetic disk. ThisBER did not have a sufficient correlation with the above-describedsurface roughness Ra, and even if the surface roughness Ra was small,the BER was high in some cases.

In view of this, an object of the present invention is to provide anindex for a surface roughness of a magnetic-disk substrate that has agood correlation with the above-described magnetic-disk property, and amagnetic-disk substrate and to provide a magnetic disk that haveexcellent magnetic-disk properties, and a method for manufacturing amagnetic disk.

One aspect of the present invention is a magnetic-disk substrate. Themagnetic-disk substrate has a pair of main surfaces, an arithmeticaverage roughness Ra of each of the main surfaces is 0.11 nm or less,and the arithmetic average roughness Ra is a value obtained throughmeasurement using an atomic force microscope provided with a probehaving a probe tip provided with a carbon nanofiber rod-shaped member.The magnetic-disk substrate is made of glass or aluminum alloy.

Another aspect of the present invention is also a magnetic-disksubstrate. The magnetic-disk substrate has a pair of main surfaces, insurface unevenness of the main surfaces, an average area of regionsoccupied by a plurality of protrusions having a height of 0.1 nm or morefrom an average plane of the surface unevenness is 25 nm²/protrusion orless, and the surface unevenness of the main surfaces is a valueobtained through measurement using an atomic force microscope providedwith a probe having a probe tip provided with a carbon nanofiberrod-shaped member. The magnetic-disk substrate being made of glass oraluminum alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the external shape of a magnetic-disksubstrate of this embodiment.

FIG. 2 is a diagram illustrating a portion of a probe including a probetip used in an atomic force microscope used to measure a magnetic-disksubstrate of this embodiment.

FIG. 3 is a diagram that two-dimensionally expresses one example of acontour curve element of the surface unevenness of a main surface of amagnetic-disk glass substrate.

FIG. 4 is a diagram showing that a surface roughness Ra measured with anatomic force microscope in which a conventional probe is used and theBER has a low correlation.

FIG. 5 is a diagram showing that a surface roughness Ra measured with anatomic force microscope in which a probe of this embodiment is used andthe BER has a high correlation.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a magnetic-disk substrate, a magnetic disk, and a methodfor manufacturing a magnetic-disk substrate of the present inventionwill be described in detail.

The inventor of this invention thought the reason why the BER and thesurface roughness Ra did not have a sufficient correlation was asfollows. That is, usually, the probe tip of the atomic force microscopeused to measure the surface roughness Ra has a pyramidal shape or aconical shape constituted by a Si (silicon) single crystal. The inventorof the invention thought that new unevenness was formed on the surfaceunevenness that was to be measured originally due to this probe tipcoming into contact with a surface of a magnetic-disk substrate, whichis the measurement target, and this new unevenness was measured as thesurface unevenness of the main surface of the magnetic-disk substrate.Thus, such information on the surface unevenness of the main surfaceincluding new unevenness formed by the probe tip does not indicate thesurface unevenness that was to be measured originally.

Also, the surface of this probe tip is oxidized and a nonconductive SiO₂layer is formed on the surface in some cases. In this case, when thesurface unevenness of a main surface of a magnetic-disk substrate ismeasured by oscillating the probe tip at a constant frequency andbringing the probe tip close to or into contact with the main surface ofthe magnetic-disk substrate, and a glass substrate is used as themagnetic-disk substrate, static electricity tends to occur at the probetip in particular, and static electricity tends to also occur on theglass substrate. As a result, there are cases where the surfaceunevenness of the glass substrate cannot be precisely measured becausethe movement of the probe tip is affected by the static electricity.Such a problem arises due to the degree of the surface unevenness of theglass substrate being made much smaller than conventional surfaceunevenness, and does not arise in the range of the surface unevenness ofa conventional glass substrate. In particular, a silicon single crystalis very hard and has the same degree of hardness as the glass substrate,and thus the above-described phenomenon tends to occur. Although theinfluence of newly formed unevenness was not recognized on the surfaceroughness of the conventional glass substrate or at the recordingdensity level, it is inferred that the main surfaces of a glasssubstrate have become ultra smooth and the recording density hasincreased dramatically, such as 750 gigabytes or more per 2.5-inchsubstrate, and 1000 gigabytes or more per 3.5-inch substrate, and thusthe glass substrate is affected by newly formed unevenness. Therefore,the magnetic-disk substrate of the present invention is particularlypreferably used in a magnetic disk having a recording density equivalentto 750 gigabytes or more per nominal 2.5-inch magnetic disk. Also, ifthe magnetic disk has a nominal 3.5-inch size, it is preferably used ina magnetic disk having a recording density equivalent to 1000 gigabytesor more per magnetic disk. Note that the magnetic-disk substrate of thepresent invention can be applied to a magnetic disk having a recordingdensity equivalent to 500 gigabytes or more per nominal 2.5-inchmagnetic disk.

Also, when an excessive force is applied to the probe tip, the formationof new unevenness on the main surfaces of the magnetic-disk substrate isreduced due to deformation of the probe tip, but the probe tip has aproperty of returning to its original shape in the case where the probetip deforms significantly. When the probe tip deforms significantly, aprobe tip that is kept in a bent state and is unlikely to return to itsoriginal shape is not preferable as the probe tip.

By measuring the surface unevenness of a main surface of a magnetic-disksubstrate using a probe having a probe tip provided with a rod-shapedmember constituted by carbon nanofibers or a rod-shaped member having aYoung's modulus of 100 GPa or less, the inventor of the presentinvention found that the measurement results and magnetic-diskproperties have a high correlation and accomplished an embodiment below.

Definition

A surface roughness Ra in this specification is an arithmetic averageroughness Ra in conformity with JIS B0601: 2013.

The surface unevenness of a main surface of a magnetic-disk substrate ismeasured using an atomic force microscope in a 1 μm×0.25 μm rectangularevaluation region with 512 points×128 points as the measurement points.

Thus, the surface roughness Ra is a value of surface unevenness in the 1μm×0.25 μm region.

In this specification, the area of regions occupied by a plurality ofprotrusions having a height of x nm (x indicates a positive number suchas 0.1 or 0.2) or more from an average plane of surface unevennessrefers to the cross-sectional area of the protrusions of a magnetic-disksubstrate that are cut when the magnetic-disk substrate is cut at aheight of x nm from the average plane of the surface unevenness of themagnetic-disk substrate. Also, in other words, when an average planecalculated from data obtained by measuring the surface unevenness of themagnetic-disk substrate is used as a reference plane and a cross sectionof this surface unevenness obtained using a plane at a height of x nmfrom the reference plane is assumed, the area refers to the area of oneor more regions obtained at this cross section.

Magnetic Disk

A central portion of a disk shape is hollowed out in a concentric shapeso that the magnetic disk has a ring plate shape, and the magnetic diskrotates about this ring during magnetic recording. The magnetic diskincludes a substrate and at least a magnetic layer. Note that inaddition to the magnetic layer, an adherent layer, a soft magneticlayer, a nonmagnetic base layer, a perpendicular magnetic recordinglayer, a protecting layer, a lubricant layer, and the like are formed,for example. A glass substrate or a substrate that is provided with aplating layer and is made of an aluminum alloy is used as the substrate.A Cr alloy or the like is used in the adherent layer, for example. Theadherent layer functions as a layer for affixing the glass substrate,and thus the adherent layer is not required in the case of a substrateobtained by forming an NiP plating layer on an aluminum alloy basematerial. A CoTaZr alloy or the like is used in the soft magnetic layer,for example. A granular nonmagnetic layer or the like is used as thenonmagnetic base layer, for example. A granular nonmagnetic layer or thelike is used as the perpendicular magnetic recording layer, for example.A material made of hydrogen carbon is used in the protecting layer. Afluorine-based resin or the like is used in the lubricant layer, forexample.

The magnetic disk is obtained by sequentially forming, on both mainsurfaces of a glass substrate, a CrTi alloy adherent layer, a CoTaZralloy soft magnetic layer, an NiW alloy seed layer, a Ru base layer, aCoCrPt—SiO₂.TiO₂ alloy first magnetic recording layer, a CoCtPtB alloysecond magnetic recording layer, and a hydrogenated carbon protectinglayer, using a single-substrate sputtering apparatus, for example.Furthermore, a perfluoropolyether lubricant layer is formed with adipping method on the outermost layer of the formed layers.

Also, if the glass substrate is used for a magnetic disk for anenergy-assisted magnetic recording method, it is preferable to use anFePt-based alloy or a CoPt-based alloy as the magnetic recording layer.

Aluminosilicate glass, soda-lime glass, borosilicate glass, and the likecan be used as a material for the magnetic-disk glass substrate used asone example of the present embodiment. In particular, aluminosilicateglass can be suitably used in light of the fact that chemicalstrengthening can be carried out, and a magnetic-disk glass substratewith excellent flatness of main surfaces and substrate strength can beproduced. Note that in the above-described viewpoint, amorphousaluminosilicate glass is more preferably used.

Also, glass having the glass composition below can be used. The glasstransition point of glass having the glass composition below ispreferably 600° C. or more, and more preferably 650° C. or more. Glasshaving a glass transition point of 600° C. or more can be suitably usedin a magnetic-disk substrate for energy-assisted magnetic recording inwhich magnetism and heat are used in combination when magnetic recordingis performed.

Glass Composition 1

The glass substrate of the present embodiment is preferably amorphousaluminosilicate glass having a composition including, in mass %, SiO₂ inan amount of 57 to 75%, Al₂O₃ in an amount of 5% to 20% (note that thetotal amount of SiO₂ and Al₂O₃ is 74% or more), ZrO₂, HfO₂, Nb₂O₅,Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ in a total amount of more than 0 and 6% orless, LiO in an amount of more than 1% and 9% or less, Na₂O in an amountof 5 to 18% (note that a mass ratio LiO/Na₂O is 0.5 or less), K₂O in anamount of 0 to 6%, MgO in an amount of 0 to 4%, CaO in an amount of morethan 0% and 5% or less (note that the total amount of MgO and CaO is 5%or less, and the content of CaO is greater than the content of MgO), andSrO+BaO in an amount of 0 to 3%.

Glass Composition 2

Also, the glass substrate of the present embodiment is preferablyamorphous aluminosilicate glass having a composition including, in termsof oxide amount in mol %, SiO₂ in an amount of 50 to 75%, Al₂O₃ in anamount of more than 0% and 15% or less, at least one component selectedfrom Li₂O, Na₂O, and K₂O in a total amount of 5 to 35%, at least onecomponent selected from MgO, CaO, SrO, BaO, and ZnO in a total amount of0 to 20%, and at least one component selected from ZrO₂, TiO₂, La₂O₃,Y₂O₃, Ta₂O₅, Nb₂O₅, and HfO₂ in a total amount of 0 to 10%. Note that ifthe glass substrate having this composition is used as the glasssubstrate used for a magnetic disk for an energy-assisted magneticrecording method, it is sufficient to adjust the glass composition asappropriate such that the glass transition point (Tg) is 600° C. ormore, for example.

If an aluminum alloy base material is used as the magnetic-disksubstrate, a base material whose surface is provided with an NiP platinglayer for increasing its surface hardness is used.

Magnetic-Disk Substrate

FIG. 1 is a diagram showing the showing the external shape of amagnetic-disk substrate on which the above-described magnetic layer andthe like of the present embodiment are not formed. In the presentembodiment, a substrate obtained by forming a NiP plating layer on aglass substrate or an aluminum alloy base material is suitably used asthe magnetic-disk substrate. These substrates can be used asmagnetic-disk substrates with a perpendicular magnetic recording methodor an energy-assisted magnetic recording method.

As shown in FIG. 1, a magnetic-disk substrate 1 in the presentembodiment is a donut-shaped thin substrate having an inner hole 2.There is no limitation on the size of the substrate. The magnetic-disksubstrate 1 can be used as a substrate having a nominal 1.8 to 3.5-inchsize, for example. There is also no particular limitation on itsthickness, and the thickness can be 0.3 to 3 mm, for example.

The magnetic-disk substrate of the present embodiment (hereinafter alsosimply referred to as a substrate) has a pair of main surfaces providedon both sides, side wall surfaces extending perpendicularly to the pairof main surfaces, and chamfered surfaces that are provided between theside wall surfaces and the pair of main surfaces, extend from the sidewall surfaces with an inclination with respect to the side wallsurfaces, and are connected to the main surfaces. The side wall surfacesand the chamfered surfaces are not shown in FIG. 1. The side wallsurfaces and the chamfered surfaces are formed at an outercircumferential side edge portion and an inner circumferential side edgeportion of the substrate. Note that a portion or all of the chamferedsurfaces may be formed into an arc shape in a sectional view.

With regard to the main surfaces on both sides of the magnetic-disksubstrate of the present embodiment, surface roughnesses Ra of the mainsurfaces are 0.11 nm or less. At this time, the surface roughness Ra isa value obtained through measurement using an atomic force microscopewith a probe having a probe tip provided with a carbon nanofiberrod-shaped member. FIG. 2 is a diagram illustrating a portion of a probeincluding a probe tip that is used in the atomic force microscope.

The tip of the probe 3 shown in FIG. 2, that is, the probe tip isprovided with a carbon nanofiber rod-shaped member 4. Unlike carbonnanotubes, carbon nanofibers are members whose inner portions are filledwith carbon atoms. Carbon nanotubes have a six-membered ring networkformed by carbon atoms, as their outer walls hollow, and their innerportions are hollow, and their perpendicular cross-sections in thelongitudinal direction have an annual ring shape (concentric shape). Inthe present embodiment, rod-shaped carbon nanofibers are used at theprobe tip. The probe 3 can be obtained by performing shape processing ona Si single crystal through etching so as to produce, as shown in FIG.2, a cantilever having a sharp pyramidal end portion, then forming acarbon film at the sharp portion of the pyramidal shape of thecantilever, emitting an ion beam of argon to this carbon film oremitting the above-described ion beam while depositing a carbon compoundthereon, and thereby forming the carbon nanofiber rod-shaped member 4 atthe sharp portion of the pyramidal shape as the probe tip.

The diameter of the carbon nanofiber rod-shaped member 4 is 3 to 60 nm,and the length thereof is 5 to 1000 nm, for example.

It is preferable that the carbon nanofiber rod-shaped member 4 has anappropriate Young's modulus and easily undergoes elastic deformation,and from the viewpoint of precisely detecting unevenness of the mainsurfaces on both sides of the substrate, the Young's modulus ispreferably 100 GPa or less, for example. In this case, the Young'smodulus is more preferably 50 GPa or less, and even more preferably 30GPa or less. In this case, although there is no particular limitation onthe lower limit of the Young's modulus, the lower limit is 1 GPa, forexample.

Note that a radius of curvature of the tip of the rod-shaped member 4 ispreferably 1 to 30 nm in light of the fact that a highly precise surfaceroughness Ra can be calculated.

Parameters for surface unevenness including the surface roughness Ra ofthe substrate are preferably obtained using the atomic force microscopeprovided with the probe having such a probe tip. Specifically, whenmeasurement is performed while the probe is moved on the main surface ofthe substrate, information on the position of the probe obtained bychanging the position of the probe in accordance with the surfaceunevenness of the main surface of the substrate such that the probeoscillating at a constant frequency oscillates at a constant amplitudeis acquired as measurement data. In the present embodiment, theparameters for surface unevenness including the surface roughness Ra ofthe main surface of the substrate are preferably obtained based on thismeasurement data. This measurement is referred to as Intermittentcontact mode. With this measurement method, the main surface of anevaluation target substrate is scanned while being tapped with a probethat oscillates vertically at a high speed, and thus it is said thatminute unevenness is easily detected on a hard surface and accuratemeasurement is possible. The surface roughness Ra and the average areaof regions occupied by a plurality of protrusions having a height of 0.1nm from an average plane of the surface unevenness, which are obtainedas the parameters for the surface unevenness through measurement, can beused as indices for the surface unevenness of a magnetic-disk glasssubstrate, the indices having a high correlation with the magnetic-diskproperties. In the measurement with this atomic force microscope, theprobe is preferably oscillated at a spring constant of 0.1 to 80 N/m anda frequency of 30 to 400 kHz in light of the fact that the indices forthe surface unevenness of the magnetic-disk substrate that has a highcorrelation with the magnetic-disk properties can be obtained. Theabove-described spring constant is more preferably 0.5 to 4 N/m. Theabove-described frequency is more preferably 50 to 100 kHz.

In the surface unevenness of the main surfaces of the magnetic-disksubstrate that are measured in this manner, the average area of regionsoccupied by a plurality of protrusions having a height of 0.1 nm or morefrom an average plane of this surface unevenness is 25 nm²/protrusion orless and preferably 20 nm²/protrusion or less, in light of the fact thatexcellent magnetic-disk properties are exhibited. Here, the averageplane is the plane that is set such that the volume of protrusionsprotruding from the average plane and the volume of recesses that arerecessed from the average plane are equal to each other.

FIG. 3 is a diagram that two-dimensionally expresses one example of acontour curve element of the surface unevenness of the main surface ofthe glass substrate. The horizontal axis in FIG. 3 expresses theposition on the main surface in a given direction along the main surfaceof the glass substrate, and the vertical axis expresses the height ofthe surface unevenness. In the following description, two-dimensionalsurface unevenness will be used instead of the three-dimensional surfaceunevenness formed on the main surface. In FIG. 3, a straight line at aposition in a height direction of 0.1 nm or more from an average line 12of the contour curve element 10 is expressed by a level line 14. Theaverage line 12 is a straight line indicating the level in the heightdirection at which the area of protrusions protruding from the averageline 12 and the area of recesses that are recessed are equal to eachother. At this time, there are three protrusions protruding from thelevel line 14, and the lengths of regions occupied by the protrusionsare S₁, S₂, and S₃. Thus, an average line segment of a region occupiedby one protrusion in the plurality of protrusions is (S₁+S₂+S₃)/3. Thus,in the present embodiment, such a line segment of the region occupied bythe protrusions of the two-dimensional surface unevenness is applied tothe three-dimensional surface unevenness formed on the main surface ofthe glass substrate.

More specifically, areas having a height of 0.1 nm or more from theaverage plane are extracted from image data of the surface unevennessmeasured using the atomic force microscope, and the number ofprotrusions having a height of 0.1 nm or more and the area of theregions occupied by the protrusions are calculated with image analysissoftware, and then the average area of the regions occupied by oneprotrusion is calculated by dividing the above-described area by thenumber. In this case, in terms of image analysis, the extracted areashaving one pixel and the extracted areas having a straight line shape(having a circularity of 0) are excluded, and the number of protrusionsand the area of the regions occupied by the protrusions are calculated.Accordingly, the area of the horizontal cross-sections of protrusions ata height of 0.1 nm from the average plane of the surface unevenness canbe obtained as the area of regions occupied by a plurality ofprotrusions having a height of 0.1 nm or more from the average plane.Similarly, areas having a height of 0.2 nm or more from the averageplane are extracted from the image data of the surface unevenness, andthe number of protrusions having a height of 0.2 nm or more and the areaof regions occupied by the protrusions are calculated with imageanalysis software, and then the average area of the regions occupied byone protrusion is calculated by dividing the above-described area by thenumber. Accordingly, the area of horizontal cross-sections ofprotrusions at a height of 0.2 nm from the average plane of the surfaceunevenness can be obtained as the area of the regions occupied by theplurality of protrusions having a height of 0.2 nm or more from theaverage plane.

In the present embodiment, the average area of the regions occupied bythe plurality of protrusions having a height of 0.2 nm or more from theaverage plane of the surface unevenness of the main surface of the glasssubstrate is more preferably 13 nm²/protrusion or less, and morepreferably 10 nm²/protrusion or less in light of the fact that excellentmagnetic-disk properties are exhibited.

As described above, in one aspect of the above-described embodiment,setting the average area of the regions occupied by the plurality ofprotrusions having a height of 0.1 nm or more from the average plane ofthe surface unevenness of the main surface of the substrate to 25nm²/protrusion or less, or preferably 20 nm²/protrusion or lesscorresponds to making the average area of the regions occupied by oneprotrusion smaller than the protrusions of a main surface of aconventional magnetic-disk substrate. As a result of this, it ispossible to reduce variation in the orientation of crystals in amagnetic layer for causing perpendicular magnetization that is formed ona main surface of the substrate as the magnetic disk. Therefore, it ispossible to reduce the BER, which is the magnetic-disk property that hasconventionally been problematic. Also, setting the average area of theregions occupied by the plurality of protrusions having a height of 0.2nm or more from the average plane of the surface unevenness of the mainsurface of the substrate to 13 nm²/protrusion or less, or preferably 10nm²/protrusion or less makes it possible to further reduce variation inthe orientation of crystals in the magnetic layer for causing theperpendicular magnetization that is formed on the main surface of thesubstrate.

In the substrate of one aspect of the present embodiment, the surfaceroughness Ra of the main surface measured using the atomic forcemicroscope with the probe having the probe tip provided with the carbonnanofiber rod-shaped member is 0.11 nm or less. The conventional probetip has a polygonal pyramidal shape or a conical shape, and the carbonnanofiber rod-shaped member is not used, and thus the probe tip formsnew unevenness on the main surface of the substrate during measurement,and information on the surface unevenness that was to be measuredoriginally tends to be buried by new unevenness. Thus, it is difficultto measure actual surface unevenness. Therefore, even if a substrate hada surface roughness Ra of 0.11 nm or less measured through conventionalmeasurement of surface unevenness, the substrate had a low BER in somecases. In the present embodiment, even if an excessive force is appliedto the probe tip and the rod-shaped member undergoes elastic deformationand warps due to use of the carbon nanofiber rod-shaped member at theprobe tip, the probe tip does not form new unevenness on the mainsurface of the substrate during measurement, and the actual surfaceunevenness can be measured accurately.

In particular, conventionally, there were no substrates in which theaverage area of the regions occupied by the plurality of protrusionshaving a height of 0.1 nm or more from the average plane of the surfaceunevenness of the main surface of the substrate was 25 nm²/protrusion orless, or preferably 20 nm²/protrusion or less, and the average area ofthe regions occupied by the above-described protrusions was greater than25 nm²/protrusion on the conventional magnetic-disk substrate.

Note that the carbon nanotubes, which have outer walls that aresix-membered ring networks formed with carbon atoms, have inner portionsthat are cavities (hollow), and have perpendicular cross sections in thelongitudinal direction that have an annual shape (concentric shape),have a regular crystalline structure with a high strength, and thus thecarbon nanotubes have a high Young's modulus and are unlikely to warp,and even if an excessive force is applied thereto, the carbon nanotubesare bent and cannot return to their original form in some cases. Thus,instead of a structure with a high Young's modulus, such as carbonnanotubes, it is preferable to use solid rod-shaped carbon nanofibersthat have a low Young's modulus, and tend to undergo elasticdeformation. Use of a rod-shaped member having a Young's modulus of 100GPa or less at the probe tip makes it possible to prevent the probe tipfrom forming new unevenness on a main surface of a glass substrateduring measurement and to accurately measure the actual surfaceunevenness.

FIG. 4 is a diagram showing that a bit error rate (hereinafterabbreviated as BER) and a surface roughness Ra (Si probe Ra) measuredwith a conventional atomic force microscope in which a Si probe having aprobe tip constituted by Si is used have a low correlation. The verticalaxis in FIG. 4 indicates x in the case where the BER is expressed as10^(x). If the BER is 10^(−5.0), for example, −5.0 is indicated in thevertical axis in FIG. 4. As shown in FIG. 4, the surface roughness Raand the BER, which is one of the magnetic-disk properties, have a lowcorrelation.

In contrast, in the present embodiment, since the rod-shaped member madeof carbon nanofibers (CNFs) is used as the probe tip, as shown in FIG.5, the surface roughness Ra (CNF probe Ra) and the BER, which is one ofthe magnetic-disk properties, has a high correlation. FIG. 5 is adiagram showing that the BER and the surface roughness Ra (CNF probe Ra)measured with the atomic force microscope in which the probe having theprobe tip provided with a CNF rod-shaped member is used have a highcorrelation. Similarly to FIG. 4, the vertical axis in FIG. 5 indicatesx in the case where the BER is expressed as 10^(x). For example, if theBER is 10^(−5.0) is indicated. In the surface unevenness of mainsurfaces of the substrate used in the examples shown in FIGS. 4 and 5,the average area of regions occupied by a plurality of protrusionshaving a height of 0.1 nm or more from the average plane of the surfaceunevenness is approximately constant.

In this manner, the present embodiment can provide indices for surfaceroughnesses of main surfaces of a magnetic-disk glass substrate, theindices having a high correlation with the BER, which is themagnetic-disk property, and can provide a magnetic-disk glass substratehaving a low BER.

Also, if a Si member is used at a probe tip as in the conventionalprobe, a SiO₂ oxide film having a low electric conductivity is formed onthe surface of the tip, and static electricity tends to remain at theprobe tip. During measurement of the surface roughness Ra, remainingstatic electricity tends to have an adverse influence on the operationof the probe. If the surface roughness Ra is measured using an atomicforce microscope with a probe having a probe tip provided with aconductive rod-shaped member, one end of the rod-shaped member ispreferably connected to earth. In this case, even if static electricitythat has an adverse influence on the probe operation occurs due tofriction between the main surface of the glass substrate and the probetip, the static electricity can be smoothly released via earth, and thusdoes not easily affect the measurement. Thus, an accurate surfaceroughness Ra can be measured.

Such a magnetic-disk glass substrate can be realized with a method formanufacturing a magnetic-disk glass substrate that will be describedbelow.

Furthermore, in the magnetic-disk substrate of the present embodiment, adifference between the average area of the regions occupied by theplurality of protrusions having a height of 0.1 nm or more from theaverage plane of the surface unevenness and the average area of theregions occupied by the plurality of protrusions having a height of 0.2nm or more from the average plane of the surface unevenness ispreferably 13 nm²/protrusion or less in order to keep a good BER.Although the reason for this is not necessarily clear, it is inferredthat the shape of protrusions on the surface of the substrate easilycoincide with each other by setting the above-described difference to 13nm²/protrusion or less, and variation in size of magnetic particles canbe suppressed when the substrate is formed into a magnetic disk. It isinferred that if there is significant variation in the shape ofprotrusions on the surface of the substrate, many magnetic particleswill grow abnormally, and as a result, noise will increase at the timeof recording reproduction and the BER will deteriorate. The surfaceunevenness of the main surfaces of the above-described magnetic-disksubstrate is a value measured using the atomic force microscope providedwith the probe having the probe tip provided with the carbon nanofiberrod-shaped member. At this time, the area occupied by the plurality ofprotrusions having a height of 0.1 nm or more from the average plane ofthe surface unevenness of the magnetic-disk substrate is preferably 20%or less, and more preferably 18% or less with respect to the area of themeasurement range (1 μm×0.25 μm). Furthermore, the number of theprotrusions having a height of 0.1 nm or more from the average plane ofthe surface unevenness of the magnetic-disk substrate is preferably 500or more, and preferably in a range of 600 or more and 800 or less in theabove-described measurement range 1 μm×0.25 μm. The number ofprotrusions having a height of 0.2 nm or more from the average plane ofthe surface unevenness is preferably 500 or less, and more preferably ina range of 150 or more and 450 or less in the above-describedmeasurement range 1 μm×0.25 μm. With the magnetic-disk substrate withlimited surface unevenness, the above-described surface roughness Ra ispreferably 0.11 nm or less, but may be greater than 0.11 nm, and theupper limit of the surface roughness Ra is preferably 0.15 nm.

Also, as described above, with the magnetic-disk substrate having asurface roughness Ra of 0.11 nm or less, the average area of the regionsoccupied by the plurality of protrusions having a height of 0.1 nm ormore from the average plane of the surface unevenness is preferably 25nm²/protrusion or less, but may be greater than 25 nm²/protrusion, andthe upper limit of the average is preferably 30 nm²/protrusion.

Method for Manufacturing Magnetic-Disk Glass Substrate

Next, a method for manufacturing a magnetic-disk glass substrate of thepresent embodiment will be described. First, a glass blank that servesas the blank for a plate-shaped magnetic-disk glass substrate having apair of main surfaces is produced. For example, the glass blank isproduced through press molding processing. Note that although the glassblank is produced through press molding in the present embodiment, aglass plate may be formed with a known float method, redraw method, orfusion method and a glass blank having the same shape as theabove-described glass blank may be cut out from the formed glass plate.Next, a circular inner hole is formed in a center portion of theproduced glass blank so as to produce a ring-shaped (annular) glasssubstrate (circular hole formation processing). Next, shape processingis performed on the glass substrate provided with the inner hole (shapeprocessing). Accordingly, the glass substrate can be obtained. Next,edge surface polishing is performed on the glass substrate (edge surfacepolishing processing). Grinding with fixed abrasive particles isperformed on the main surfaces of the glass substrate that was subjectedto edge surface polishing (grinding processing). Next, polishing isperformed on the main surfaces of the glass substrate (polishingprocessing). Polishing is performed a plurality of times in thepolishing processing. Chemical strengthening may be performed as neededon the glass substrate during the plurality of times of polishing(chemical strengthening processing). Thereafter, ultrasonic cleaning isperformed on the glass substrate that has undergone the polishingprocessing. A magnetic-disk glass substrate can be obtained through theabove-described processes. Hereinafter, the respective processes will bedescribed in detail.

(a) Press Molding Processing

A glass blank is molded by cutting molten glass flow with a cutter,sandwiching the mass of the cut molten glass between press moldingsurfaces of a pair of metal molds, and pressing the mass. After pressingis performed for a predetermined amount of time, the metal molds areopened to remove the glass blank.

(b) Circular Hole Formation Processing

A disk-shaped glass substrate provided with a circular hole is obtainedby forming a circular inner hole in the glass blank using a drill or thelike.

(c) Shape Processing

In the shape processing, chamfering processing is performed on edgesurfaces of the glass substrate that has undergone the circular holeformation processing. Chamfering processing is performed using agrinding grindstone or the like. On the edge surfaces of the glasssubstrate, side wall surfaces of the substrate that extendperpendicularly to the main surfaces of the glass substrate and edgesurfaces that are provided between these side wall surfaces and the mainsurfaces and extend with an inclination with respect to the side wallsurfaces are formed through chamfering processing.

(d) Edge Surface Polishing Processing

In the edge surface polishing processing, mirror finishing is performedon an inner circumferential side edge surface and an outercircumferential side edge surface of the glass substrate through edgesurface polishing processing using a polishing liquid containingpolishing abrasive particles.

(e) Grinding Processing

In the grinding processing, grinding is performed on the main surfacesof the glass substrate using a double-side grinding apparatus providedwith a planetary gear mechanism. Specifically, the main surfaces on bothsides of the glass substrate are ground while the outer circumferentialside edge surface of the glass substrate is held in a holding holeprovided in a holding member of the double-side grinding apparatus. Thedouble-side grinding apparatus has a pair of upper and lower surfaceplates (an upper surface plate and a lower surface plate), and the glasssubstrate is sandwiched between the upper surface plate and the lowersurface plate. Moreover, the glass substrate and the surface plates aremoved relative to each other by moving one or both of the upper surfaceplate and the lower surface plate, and thereby both main surfaces of theglass substrate can be ground.

(f) Polishing Processing

Next, polishing is performed on the ground main surfaces of the glasssubstrate. Specifically, the main surfaces on both sides of the glasssubstrate are polished while the outer circumferential side edge surfaceof the glass substrate is held in a holding hole provided in a carrierfor polishing of the double-side grinding apparatus. Polishing isperformed using a double-side polishing apparatus. In the double-sidepolishing apparatus, the glass substrate is held between the pair ofupper and lower surface plates. Tabular polishing pads (resin polisher,for example) having an annular shape overall are attached to the uppersurface of the lower surface plate and the bottom surface of the uppersurface plate. The glass substrate and the surface plates are movedrelative to each other by moving one or both of the upper surface plateand the lower surface plate, and thereby both main surfaces of the glasssubstrate are polished.

Polishing is preferably performed three times; first to third polishing,in light of the fact that the surface roughnesses Ra of the mainsurfaces of the glass substrate of the present embodiment are 0.11 nm orless or the average area of the regions occupied by the plurality ofprotrusions having a height of 0.1 nm from the average plane of thesurface unevenness is 25 nm²/protrusion or less.

First polishing is performed in order to remove blemishes and warpingand adjust minute surface unevenness (microwaviness, roughness)remaining on the main surfaces that have undergone the grindingprocessing, and in order to mirror-polish the main surfaces. Ceriumoxide abrasive particles, zirconia abrasive particles, or the like, forexample, are used, and are supplied between the polishing pads of thedouble-side polishing apparatus and the main surfaces of the glasssubstrate, and thus the main surfaces of the glass substrate arepolished. The polishing abrasive particles used in first polishing arecerium oxide particles having an average particle diameter d50 of 0.1 to1.5 μm, for example.

Second polishing is performed for mirror-polishing for further smoothingthe main surfaces of the glass substrate. In second polishing, apolishing liquid containing colloidal silica having an average particlediameter d50 of 5 to 20 nm as loose abrasive particles is suppliedbetween the polishing pads of the double-side polishing apparatus andthe main surfaces of the glass substrate, and the main surfaces of theglass substrate are polished. The pH of the polishing liquid ispreferably acidic and more preferably 1 to 4 from the viewpoint of apolishing rate and a reduction in foreign matter remaining on thesurfaces that have undergone polishing. It is preferable that thepolishing machining allowance is 1 to 5 μm in terms of the substratethickness (the total amount of machining allowance for both mainsurfaces).

In third polishing, the degree of smoothness of the main surfaces of theglass substrate is further increased. In third polishing, a polishingliquid containing colloidal silica having an average particle diameterd50 of 30 to 100 nm as loose abrasive particles is supplied between thepolishing pads of the double-side polishing apparatus and the mainsurfaces of the glass substrate, and the main surfaces of the glasssubstrate are polished. The pH of the polishing liquid is preferablyalkaline and more preferably 11 to 13 from the viewpoint of smoothness.It is preferable that the machining allowance is 0.05 to 1 μm in termsof the substrate thickness.

Also, it is preferable that an acidic polishing liquid is used in secondpolishing, and an alkaline polishing liquid is used in third polishing.It is preferable to perform polishing using the alkaline polishingliquid after polishing using an acidic liquid in light of the fact thatthe alkaline polishing liquid has a slower polishing speed compared tothe acidic polishing liquid and can realize highly precise polishing.

Also, as described above, it is preferable that the average particlediameter d50 of colloidal silica in third polishing is larger than insecond polishing. The average particle diameter d50 of colloidal silicain third polishing is more preferably two times or more and even morepreferably three times or more the average particle diameter d50 insecond polishing. Doing so makes it possible to sufficiently remove,with silica abrasive particles having relatively large particlediameters, minute and sharp single-cuts formed on the surfaces of thesubstrate by silica abrasive particles having small particle diameters,and to form the surfaces of a magnetic-disk substrate of the presentembodiment. It is conceivable that if the shape such as so-called burr,which is minute and sharp, remains on the surface of the substrate, thesubstrate is likely to break when measurement is performed with a probehaving a high hardness, such as a single crystal Si probe.

Also, the machining allowance in third polishing is preferably smallercompared to that in second polishing. It is preferable that thepolishing time of third polishing is ⅓ or less and ¼ or less thepolishing time of second polishing, for example, in light of the factthat the above-described surface roughness Ra is 0.11 nm or less, or theaverage area of the regions occupied by the plurality of protrusionshaving a height of 0.1 nm or more from the average plane of the surfaceunevenness is 25 nm²/protrusion or less.

Note that the machining allowance for the glass substrate in first tothird polishing is 30 μm or less, preferably 25 μm or less, and morepreferably 20 μm.

In this manner, when two-step polishing with colloidal silica isperformed in the final stage of polishing processes, the pH of thepolishing liquid, the particle diameter of colloidal silica, and themachining allowance are combined so as to be respectively in theabove-described ranges, and thereby a magnetic-disk glass substrate thatis suitable for the present invention can be obtained.

Accordingly, the surface roughnesses Ra of the main surfaces obtainedwhen being measured using the atomic force microscope with the probehaving the probe tip provided with the carbon nanofiber rod-shapedmember can be set to 0.11 nm or less. Also, as described above, theaverage area of the regions occupied by the plurality of protrusionshaving a height of 0.1 nm or more from the average plane of surfaceunevenness of the magnetic-disk substrate can be set to 25cm²/protrusion or less by adjusting the type and particle diameter ofpolishing abrasive particles and the pH of the polishing liquid andsetting the machining allowance for the glass substrate in polishing to30 μm or less. That is, the pair of main surfaces of the substrate arepolished with a machining allowance of 30 μm or less such that theabove-described average is 25 nm²/protrusion or less.

(g) Chemical Strengthening Processing

If the glass substrate is chemically strengthened, a melt obtained bymixing potassium nitrate and sodium sulfate or the like, for example, isused as a chemical strengthening liquid, and the glass substrate isimmersed in the chemical strengthening liquid. The chemicalstrengthening processing is performed between first polishing and secondpolishing, for example. Note that the chemical strengthening processingis not essential, and need not be performed.

In this manner, a magnetic-disk glass substrate is produced.

Method for Manufacturing a Magnetic-Disk Aluminum Alloy Substrate

Next, a method for manufacturing a magnetic-disk aluminum alloysubstrate of the present embodiment will be described simply.

(a) Production of Grind Substrate

An aluminum alloy is melted, cast, rolled, and cut into a disk-shapedaluminum magnesium alloy (aluminum alloy) blank, and then cuttingprocessing is performed on inner and outer edge surfaces, and grindingis performed on the surfaces so as to produce an annular grind substrate(base material).

(b) Plating Layer Formation

Next, an aluminum alloy substrate is obtained by performing NiP platingon the surfaces of the grind substrate (base material). The aluminumalloy is light in weight and has excellent processability, but itssurface hardness does not sufficiently meet the requirements for amagnetic disk, and thus in order to prevent impairment of the substrate,an NiP plating layer is formed on the surfaces of the grind substrate.The thickness of the NiP plating layer is around a dozen μm, forexample. Furthermore, in order to reduce inner stress of the NiP platinglayer, annealing is performed.

(c) Polishing

Polishing for removing waviness and microwaviness and smoothing thegrind substrate is performed on the grind substrate provided with theplating layer. In polishing, the NiP plating layer is polished in aplurality of stages, using polishing pads (for example, resin polishers)and a polishing slurry obtained by dispersing loose abrasive particlescontaining alumina abrasive particles.

The substrate polished in this manner is cleaned so as to obtain amagnetic-disk substrate.

In this polishing as well, similarly to polishing in the above-describedmethod for manufacturing a glass substrate, the surface roughnesses Raof the main surfaces can be set to 0.11 nm or less by performing aplurality of stages of polishing. Also, the pair of main surfaces of thesubstrate can be polished with a machining allowance of 30 μm or lesssuch that the average area of the regions occupied by the plurality ofprotrusions having a height of 0.1 nm or more from the average plane ofthe surface unevenness is 25 nm²/protrusion or less.

Note that the condition that the average area of the regions occupied bythe plurality of protrusions having a height of 0.1 nm or more from theaverage plane of the above-described surface unevenness is 25nm²/protrusion or less can be applied to a step of determining whetheror not magnetic-disk properties of a substrate are in the allowableranges as a magnetic-disk substrate, in the method for manufacturing amagnetic-disk substrate.

For example, the surface roughnesses of the main surfaces of thesubstrate are measured using an atomic force microscope with a probehaving a probe tip provided with a carbon nanofiber rod-shaped member,for example, a rod-shaped member having an Young's modulus of 100 GPa orless, and the average area of regions occupied by a plurality ofprotrusions having a height of 0.1 nm or more from the average plane ofthe surface unevenness of the main surfaces of the substrate is obtainedbased on data of the surface unevenness obtained through measurement. Itis determined whether or not the obtained average area is 25nm²/protrusion or less, and if it is determined that the above-describedaverage area is 25 nm²/protrusion or less, the measured substrate isadopted as a magnetic-disk substrate. In this case, the average valueused in the above-described determination is preferably 20nm²/protrusion or less. Furthermore, the condition that the average areaof regions occupied by a plurality of protrusions having a height of 0.2nm or more from the average plane of the surface unevenness of the mainsurfaces is 12 nm²/protrusion or less is preferably used in theabove-described determination.

Such a method is performed with an atomic force microscope and anarithmetic apparatus that obtains the above-described average area basedon the data of surface unevenness obtained with the atomic forcemicroscope.

In this case as well, the condition that the surface roughness Ra thatcan be obtained based on the data of surface unevenness obtained throughthe above-described measurement is 0.11 nm or less may be included inthe conditions of the above-described determination.

Of course, the above-described screening method can be applied to themagnetic-disk glass substrate and the magnetic-disk aluminum alloysubstrate.

The magnetic-disk properties of the magnetic disk produced using such amagnetic-disk substrate are increased, and for example, the BER can beset to 10^(−5.8) or less.

Experimental Example 1

In order to check the effects of the present embodiment, 18 glasssubstrates of six types were produced by changing the conditions ofsecond polishing and third polishing, using the above-described methodfor manufacturing a glass substrate. Three substrates per type wereproduced. Glass for the glass substrates had the above-described glasscomposition.

The surface unevenness of each of the six types of glass substrates weremeasured with an atomic force microscope using a probe having a probetip provided with a carbon nanofiber rod-shaped member having a diameterof 20 nm and a length of 200 nm. Measurement was performed under thefollowing conditions.

Measurement area: 1 μm×0.25 μm rectangular region

Measurement points: 512 points×128 points (512 points for 1 μm and 128points for 0.25 μm)

Measurement mode: Intermittent contact mode

Frequency: 70 KHz

Spring constant of probe: 4 N/m

Furthermore, the surface unevenness of one of the six types of glasssubstrates was measured with an atomic force microscope using apyramidal probe having a probe tip constituted by Si. Measurement wasperformed under the above-described conditions.

Furthermore, as described above, a magnetic-disk substrate was producedby providing a magnetic layer and the like on the one remaining glasssubstrate in the various produced glass substrates, the magnetic-disksubstrate was integrated into a HDD (hard disk drive apparatus) togetherwith a magnetic head provided with a DFH (disk flying height) mechanism,and the BER value of the produced magnetic disk was obtained under thecondition that a protruding amount of the DFH element portion was usedas a reference, the protruding amount being obtained when the tip of theDFH element portion came into contact with the surface of the substrateby making the DFH element portion gradually protrude, and the protrudingamount was reduced by 1 nm, that is, under the condition that theback-off amount was 1 nm. Note that the linear recording density duringsignal recording was 93 kbit/mm.

Table 1 below shows the results of evaluation of the six types of glasssubstrates. The six types of glass substrates in Table 1 are expressedas Samples 1 to 6. The field “Surface roughness Ra (CNF probe Ra)” showsthe results measured using carbon nanofibers at the probe tip, and thefield “Surface roughness Ra (Si probe Ra)” shows the results measuredusing a pyramidal Si at the probe tip. Numerical values x in the field“BER” in Table 1 express the x in 10^(x).

TABLE 1 Surface roughness Ra Surface roughness Ra (Si probe Ra) (CNFprobe Ra) BER Sample 1 0.107 0.065 −6.5 Sample 2 0.168 0.102 −6.1 Sample3 0.131 0.110 −6.0 Sample 4 0.085 0.115 −5.8 Sample 5 0.143 0.137 −5.2Sample 6 0.148 0.201 −4.0

The results shown in Table 1 above are also the results shown in FIGS. 4and 5.

In this manner, as is understood from Table 1 and FIGS. 4 and 5, thesurface roughness Ra (CNF probe Ra) has a high correlation with the BER,compared with the surface roughness Ra (Si probe Ra). Furthermore, it isunderstood from Table 1 that the BER can be set to 10^(−5.8) or less andpreferably 10^(−6.0) or less, which are in the allowable range formagnetic properties, by setting the surface roughness Ra (CNF probe Ra)to 0.11 nm or less. At that time, the average area of the regionsoccupied by the plurality of protrusions having a height of 0.1 nm ormore from the average plane of surface unevenness of the glasssubstrates of Samples 1 to 6 were 30 nm²/protrusion or less.

Experimental Example 2

Furthermore, in order to check the effects of the present embodiment,eight glass substrates of four types were produced using theabove-described method for manufacturing a glass substrate by changingthe conditions of second polishing and third polishing. Glass for theglass substrates had the above-described glass composition.

The surface unevenness of each of the four types of glass substrateswere measured with an atomic force microscope using a probe having aprobe tip provided with a carbon nanofiber rod-shaped member having adiameter of 20 nm and a length of 200 nm. Measurement conditions werethe same as those in Experimental Example 1.

Furthermore, a magnetic-disk substrate was produced by providing amagnetic layer and the like on the one remaining glass substrate in thevarious produced glass substrates, the magnetic-disk substrate wasintegrated into a HDD (hard disk drive apparatus) together with amagnetic head provided with a DFH (disk flying height) mechanism, andthe BER value of the magnetic disk was obtained under the sameconditions as those in Experimental Example 1.

Table 2 below shows the results of evaluation of the four types of glasssubstrates. In Table 2, the field “0.1 nm or more” indicates the averagearea of regions occupied by a plurality of protrusions having a heightof 0.1 nm or more from the average plane of the surface unevenness, andthe field “0.2 nm or more” indicates the average area of regionsoccupied by a plurality of protrusions having a height of 0.2 nm or morefrom the average plane of surface unevenness. Numerical values x in thefield “BER” in Table 2 express the x in 10^(x).

TABLE 2 0.1 nm or more 0.2 nm or more BER Sample 7 20 nm²/protrusion 10nm²/protrusion −6.0 Sample 8 24 nm²/protrusion 12 nm²/protrusion −5.8Sample 9 28 nm²/protrusion 14 nm²/protrusion −5.5 Sample 10 33nm²/protrusion 17 nm²/protrusion −5.0

The surface roughnesses Ra of the glass substrates of Samples 7 to 10were 0.15 nm or less.

It is understood from Table 2 that Samples 7 and 8 in which the averagearea of the regions occupied by the plurality of protrusions having aheight of 0.1 nm from the average plane of the surface unevenness was 25nm²/protrusion or less had a BER of 10-5-8 or less and the magnetic-diskproperties were increased.

Although a magnetic-disk glass substrate, a method for manufacturing amagnetic-disk substrate, and a determination method for a magnetic-disksubstrate of the present invention were described in detail above, thepresent invention is not limited to the above-described embodiment, andit will be appreciated that various improvements and modifications canbe made without departing from the gist of the present invention.

According to one aspect of the embodiment, the magnetic-disk substratehas a pair of main surfaces, arithmetic average roughnesses Ra of themain surfaces are each 0.11 nm or less, and the surface roughness Ra isa value obtained through measurement using an atomic force microscopeprovided with a probe having a probe tip provided with a carbonnanofiber rod-shaped member.

According to another aspect of the embodiment, the magnetic-disksubstrate has a pair of main surfaces, in surface unevenness of the mainsurfaces, an average area of regions occupied by a plurality ofprotrusions having a height of 0.1 nm or more from an average plane ofthe surface unevenness is 25 nm²/protrusion or less, and the surfaceunevenness of the main surfaces is a value obtained through measurementusing an atomic force microscope provided with a probe having a probetip provided with a carbon nanofiber rod-shaped member.

It is preferable that the average area of the regions occupied by theplurality of protrusions having a height of 0.1 nm or more from theaverage plane of the surface unevenness is 20 nm²/protrusion or less.

It is preferable that a difference between the average area of theregions occupied by the plurality of protrusions having a height of 0.1nm or more from the average plane of the surface unevenness and anaverage area of regions occupied by a plurality of protrusions having aheight of 0.2 nm or more from the average plane of the surfaceunevenness is 13 nm²/protrusion or less.

It is preferable that the carbon nanofiber rod-shaped member provided atthe probe tip has a Young's modulus of 100 GPa or less.

It is preferable that the rod-shaped member is electrically conductive,and one end of the rod-shaped member is connected to earth.

It is preferable that the surface roughness Ra or the surface unevennessis obtained based on information on a position of the probe that isobtained by changing the position of the probe in accordance with thesurface unevenness of the main surfaces such that the probe oscillatesat a constant amplitude.

It is preferable that in the measurement performed using the atomicforce microscope, the probe is oscillated at a spring constant of 0.1 to80 N/m and a frequency of 30 to 400 KHz.

It is preferable that the substrate is a magnetic-disk substrate forenergy-assisted magnetic recording.

According to another aspect of the embodiment is a magnetic disk inwhich at least a magnetic film is formed on a surface of themagnetic-disk substrate.

Yet another aspect of the embodiment is a method for manufacturing amagnetic-disk substrate. In this manufacturing method, after grindingprocessing, in a polishing processing step, a pair of main surfaces ofthe substrate are polished with a machining allowance of 30 μm or lesssuch that an average area of regions occupied by a plurality ofprotrusions having a height of 0.1 nm or more from an average plane ofsurface unevenness of the main surfaces is 25 nm²/protrusion or less,the surface unevenness having been measured using an atomic forcemicroscope provided with a probe having a probe tip provided with acarbon nanofiber rod-shaped member.

It is preferable that the polishing processing step includes: performingfirst polishing on the pair of main surfaces of the magnetic-disksubstrate using an acidic polishing liquid; performing second polishingon the main surfaces of the magnetic-disk substrate using an alkalinepolishing liquid after the first polishing; and shortening a polishingtime in the second polishing, compared to that in the first polishing.

It is preferable that in the surface unevenness of the main surfaces,the average area of the regions occupied by the plurality of protrusionshaving a height of 0.1 nm or more from the average plane of the surfaceunevenness is 20 nm²/protrusion or less.

It is preferable that in the surface unevenness of the main surfaces, anaverage area of regions occupied by a plurality of protrusions having aheight of 0.2 nm or more from the average plane of the surfaceunevenness is 12 nm²/protrusion or less.

Furthermore, still another aspect of the embodiment is a method formanufacturing a magnetic-disk substrate including a step of determiningsurface unevenness of main surfaces of a magnetic-disk substrate anddetermining whether or not the magnetic-disk substrate is acceptable asa magnetic-disk substrate. In the determination step, the surfaceunevenness of the main surfaces of the magnetic-disk substrate ismeasured using an atomic force microscope provided with a probe having aprobe tip provided with a carbon nanofiber rod-shaped member, and basedon data of the surface unevenness obtained through measurement, in thesurface unevenness of the main surfaces, an average area of regionsoccupied by a plurality of protrusions having a height of 0.1 nm or morefrom an average plane of the surface unevenness is obtained, and if theaverage area is 25 nm²/protrusion or less, a determination that themeasured substrate is to be adopted as a magnetic-disk substrate ismade.

The condition that the average area of the regions occupied by theplurality of protrusions having a height of 0.1 nm or more from theaverage plane of the surface unevenness of the main surfaces is 20nm²/protrusion or less is preferably used in the determination.

The condition that an average area of regions occupied by a plurality ofprotrusions having a height of 0.2 nm or more from the average plane ofthe surface unevenness of the main surfaces is 12 nm²/protrusion or lessis preferably included in the determination.

According to the above-described magnetic-disk substrate, magnetic disk,and method for manufacturing a magnetic-disk substrate, it is possibleto provide a magnetic-disk substrate that have excellent magnetic-diskproperties.

1. A magnetic-disk substrate, comprising: a pair of main surfaces, insurface unevenness of the main surfaces, an average area of regionsoccupied by a plurality of protrusions having a height of 0.2 nm or morefrom an average plane of the surface unevenness being 13 nm²/protrusionor less, and the surface unevenness of the main surfaces being a valueobtained through measurement using an atomic force microscope providedwith a probe having a probe tip provided with a carbon nanofiberrod-shaped member.
 2. The magnetic-disk substrate according to claim 1,wherein an arithmetic average roughness Ra of the main surfaces is 0.15nm or less.
 3. The magnetic-disk substrate according to claim 1, whereinthe magnetic-disk substrate has a nominal 3.5-inch size.
 4. Themagnetic-disk substrate according to claim 1, wherein the magnetic-disksubstrate is a magnetic-disk substrate for energy-assisted magneticrecording.
 5. The magnetic-disk substrate according to claim 1, whereinthe magnetic-disk substrate is a magnetic-disk glass substrate made ofglass, and a glass transition point of glass of the magnetic-disk glasssubstrate is 600° C. or more.
 6. The magnetic-disk substrate accordingto claim 1, wherein a difference between an average area of regionsoccupied by a plurality of protrusions having a height of 0.1 nm or morefrom the average plane of the surface unevenness and the average area ofregions occupied by the plurality of protrusions having the height of0.2 nm or more from the average plane of the surface unevenness is 13nm²/protrusion or less.
 7. A magnetic disk, wherein at least a magneticfilm is formed on a surface of the magnetic-disk substrate according toclaim
 1. 8. A magnetic disk, wherein at least a magnetic film is formedon a surface of the magnetic-disk substrate according to claim
 2. 9. Amagnetic disk, wherein at least a magnetic film is formed on a surfaceof the magnetic-disk substrate according to claim
 3. 10. A magneticdisk, wherein at least a magnetic film is formed on a surface of themagnetic-disk substrate according to claim
 4. 11. A magnetic disk,wherein at least a magnetic film is formed on a surface of themagnetic-disk substrate according to claim
 5. 12. A magnetic disk,wherein at least a magnetic film is formed on a surface of themagnetic-disk substrate according to claim
 6. 13. A magnetic-disksubstrate, comprising: a pair of main surfaces, in surface unevenness ofthe main surfaces, a difference between an average area of regionsoccupied by a plurality of protrusions having a height of 0.1 nm or morefrom an average plane of the surface unevenness and an average area ofregions occupied by a plurality of protrusions having a height of 0.2 nmor more from the average plane of the surface unevenness being 13nm²/protrusion or less, and the surface unevenness of the main surfacesbeing a value obtained through measurement using an atomic forcemicroscope provided with a probe having a probe tip provided with acarbon nanofiber rod-shaped member.
 14. The magnetic-disk substrateaccording to claim 13, wherein an arithmetic average roughness Ra of themain surfaces is 0.15 nm or less.
 15. The magnetic-disk substrateaccording to claim 13, wherein the magnetic-disk substrate has a nominal3.5-inch size.
 16. The magnetic-disk substrate according to claim 13,wherein the magnetic-disk substrate is a magnetic-disk substrate forenergy-assisted magnetic recording.
 17. The magnetic-disk substrateaccording to claim 13, wherein the magnetic-disk substrate is amagnetic-disk glass substrate made of glass, and a glass transitionpoint of glass of the magnetic-disk glass substrate is 600° C. or more.18. A magnetic disk, wherein at least a magnetic film is formed on asurface of the magnetic-disk substrate according to claim
 13. 19. Amagnetic disk, wherein at least a magnetic film is formed on a surfaceof the magnetic-disk substrate according to claim
 14. 20. A magneticdisk, wherein at least a magnetic film is formed on a surface of themagnetic-disk substrate according to claim 15.